Ikaros activity in the hemopoietic stem cell (HSC) compartment is required for normal hemopoietic lineage restrictions. Ikaros, is encoded by a large and complex gene with a number of regulatory regions. Our research program is focused on the transcriptional regulation of Ikaros, and is structured around three specific aims. In the first specific aim, we seek to characterizethe activity of Ikaros regulatory regions in the HSC and early progenitor populations. Particular emphasis is given on the ability of Ikaros regulatory element reporters to identify functionally distinct progenitors. These studies are initially pursued in the wild type and will be extended to an Ikaros null setting to enable dissection and characterization of mutant progenitors impossible to achieve with conventional markers. In the second specific aim, we delineate the minimal regulatory elements with activity in the HSC/progenitor compartment. These studies seek on one hand to identify the minimal units sufficient for expression in the HSC and early progenitor populations and on the other to determine their role in the context of the intact Ikaros locus. In the third specific aim, we seek to identify the transcription inputs responsible for Ikaros regulatory element activity in the HSC and its progenitors. An RNAi approach is proposed to modulate Ikaros reporter expression in early progenitor populations and identify the transcriptional factor inputs responsible for element activity in these cells. These studies will be combined with more conventional biochemical approaches in progenitor cell lines or in more differentiated tissues. t The proposed studies on Ikaros regulatory units and their transcriptional factor input hold great promise for providing us with the genetic codes that guide transitions through the early hemopoietic hierarchy. We hypothesize that achieving some of these goals would dramatically increase our ability to analyze and manipulate development of the hemopoietic stem cell in vivo and in vitro with important therapeutic implications.